This chapter addresses how Theodosius Dobzhansky's approach to evolutionary biology undergirded his ideas about human evolution, including race. Dobzhansky's own preoccupation was with how much genetic variation exists in populations. He argued that most "gene pools" – his term – contain any number of alleles that can slot into chromosomal loci and that the frequency of particular alleles waxes or wanes as natural selection keeps populations tuned to changing environments. Dobzhansky's characteristic figure of argumentation is the argument to and from definition. In the context of global decolonization, the revelation of the horrors of the Holocaust, and the stirrings of the Civil Rights Movement in America, the anti-racist consensus that Dunn, Dobzhansky, and their anthropological allies hoped to catalyze on biological grounds began to influence public discourse. Dobzhansky's aim in writing Mankind Evolving was to support social, political, and cultural practices that let nature do its work and avoid those that hinder it.